The invention described herein may be made, used, or licensed by or for the Government for Governmental purposes without the payment to me of any royalties thereon or therefore.
The present invention relates to impregnated, surface coated wood products. More particularly, the present invention relates to the use of isocyanate-based polymers that are highly lightfast and de-contaminatable to impregnate and surface coat solid wood products with tight film formation via polymer grafting on wood cell surfaces when the prepolymers cure.
An important goal in wood research has been to improve performance properties of wood for use in harsh environments and demanding conditions comparable to those which steel is exposed to for use for various reasons. One specific case of interest is the decontaminability of various chemical warfare agents that use a strong alkaline solution. The decontamination performance quality, especially that of the chemical warfare agent VX due to its unique affinity to many surfaces, has been considered to be highly necessary in military uses of wood for pallets and other packaging components. Use of raw or surface-coated wood has been severely limited in this application because of the inherently de-contaminability arising from the porosity of wood, inadequate performance of available surface coatings materials, and environmental degradation, especially from sunlight, of wood and surface coating materials in general.
As of now, surface coatings materials for wood that can meet the relevant U.S. Military requirements are not available. The coatings approach has also been considered to be of limited utility because of the likelihood of abrasion in use, suggesting that wood properties through the entire wood thickness need to be improved. Accordingly, materials and method for impregnating and coating wood to attain lightfast and de-contaminatable properties have been a highly desirable development objective in military logistics. Convention wood impregnation prepolymers such as phenon-formaldehyde or melamine-formaldehyde were shown to have effects of decreasing the wood strength due to the brittle nature of the polymer structures. These prepolymers were unacceptable in addition due the unknown de-contaminability of the warfare agent VX on wood with these polymers impregnated therein.
One object of the present invention is therefore to provide wood impregnation and coatings materials for enhancing various wood properties including the decontaminability of the chemical warfare agent VX before and after a long exposure to the degradation effect of sunlight.
Another object of the present invention is to provide impregnation and coating procedures for manufacturing the treated wood products using the impregnation and coatings materials of this invention.
Still another object of the present invention is to provide various impregnated and coated wood materials that have various enhanced properties including the de-contaminability of the chemical warfare agent VX before and after long exposure to the degradation effect of sunlight.
Other Objects will appear hereinafter.
It has now been discovered that the above and other objects of the present invention may be accomplished in the following manner. Specifically, the invention comprises a method and process for treating wood.
The steps of the method include preparing isocyanate-based reactive prepolymer resins for impregnation, impregnating wood with these impregnation resins, curing the impregnated wood at elevated temperatures, preparing isocyanate resin-based coating materials and applying these coating materials on the cured impregnated wood and curing the coatings at room or elevated temperatures. The final product is novel and useful in a variety of applications including those applications described herein.
As noted above the present invention involves a series of steps to accomplish the objects of the invention. The first step comprises the preparation of isocyanate-based reactive prepolymer resins for wood impregnation. This may be accomplished using various commercially available isocyanate materials such as toluene diisocyanates (TDI), methylene-bisphenylene diisocyanates (MDI), and low molecular weight derivatives of MDI or other aliphatic isocyanates. All are suitable for the present invention.
Preferred are those materials that have a functionality close to 2.0. Higher functionality isocyanates can be used, but they are less suitable for synthesis of the impregnation prepolymers of this invention.
Polyols suitable for reaction with these isocyanates are linear polyols having a molecular weight range from about 500 to about 5000 daltons and a hydroxyl group functionality of about 2.0. Also polyols having a major amount of hydrophobic segments, such as propylene oxide or butadiene groups, are preferred over those of the more hydrophilic groups, such as ethylene oxide groups. Polyols suitable for this invention are linear polyols, such as, for example, ethylene oxide-propylene oxide copolymers, polytetrahydrofurans, and hydroxyl group-terminated polybutadienes.
For the synthesis of the prepolymer of this invention, the isocyanate/polyol ratio should be in a range that will keep the amount of isocyanate (NCO) functional groups, left unreacted at the end of the synthesis reaction, at a level between about 3% and 20% by weight based on the total weight of the isocyanate and and polyol materials used in the reaction. This excess isocyanate functionality is designed for later reactions with hydroxyl groups and moisture in the wood during the curing process, so that a tightly coupled wood-polymer composite material is obtained. Too high levels of isocyanate group are not desirable because of the slow curing rate in the wood and also because of the rough cured surfaces caused by excessive curing with the moisture in the air.
The prepolymer resin synthesis reaction must be conducted in the absence of moisture, preferably under a stream of dry nitrogen, and at an elevated temperature until all hydroxyl groups of the polyol are reacted. This completeness of reaction is determined when no further increase in viscosity is occurring. The reaction mixture is then cooled to room temperature and may at that time be diluted with a suitable water-dry solvent such as methyl ethyl keytone or Cellosolve Acetate, or the like. The mixture should be stored in a dry, closed container.
The second step comprises the impregnation of wood with the prepolymer resins, followed by curing them. The wood should first be dried to a moisture content ranging from about 2% to 14%, based on the oven dry wood weight and the free isocyanate group content of the prepolymer resin should be adjusted to approximate the wood moisture content level. Any wood impregnation procedure may be used as long as the impregnation is sufficiently complete. One such process is generally described as a xe2x80x9cfull cellxe2x80x9d process, used here to illustrate the process using the prepolymers of the present invention.
A wood sample is placed on a plan in a cylinder and a vacuum is applied to the cylinder, up to about 28 inches of vacuum. A prepolymer is introduced to the pan through an opening to completely cover the wood sample. The vacuum is then continued for about 30 minutes and, after release of the vacuum, a positive pressure of about 150 psi is applied for about 60 minutes. After the pressure is released, the wood sample is taken out, wiped and dried in a ventilated hood for several hours or more. Samples can then be cured in a heated oven, at about 40xc2x0 C. for 20 to 24 hours, more or less.
The third step includes preparation of surface coating materials for use in the present invention. Two package formulations based on aliphatic isocyanates and multi-functional polymeric polyols are preferred for this invention. Aliphatic isocyanates such as hexane diisocyanate and isophorone diisocyanate or their oligomeric adduct products are suitable. The isocyanate is formulated as one part of the coating system with suitable solvents for adjustment of the viscosity and other properties. The multi-functional polyol is the major component of the second part of the coating system, again with suitable solvents for adjustment of the viscosity. It is preferred that the polyol should be polymeric molecules with a molecular weight range from about 500 to about 15,000 daltons and having more than two hydroxyl groups per molecule with a background structure of either aliphatic or aromatic carbon chains.
Various additives that are normally needed for many Coating formulations may be added to this polyol part if desired. The additives particularly useful for the coating system of this invention are flow promoters such as certain acrylic copolymers, hydroxy group-crosslinking additives such as methylolated melamine-formaldehyde resins, hydrophobicizing pigment and crosslinking agents such as epoxyalkylsilanes, and graft promoters between the polymer and wood surface such as acrylic ester monomers with attendant initiators.
The hydroxyl groups of the polyol are mostly cured with the isocyanate groups by forming urethane bonds but also they are cured by forming bonds with the melamine formaldehyde and epoxysilane components. The two part coating system should be mixed well before its application in a proportion such that the number of isocyanate functionality should be somewhat higher than the number of available hydroxyl groups. This is done to assure adequate self-crosslinking by reacting with moisture in the air and for forming urea bonds. Excess isocyanate functionality as well as any deficiency of it in this final mixed coating system should be avoided.
The next step is the application of the surface coating materials followed curing it. The surface coating materials prepared as described above should be applied within an hour or two of preparation. The viscosity may be adjusted to some extend by adding a solvent such as methyl ethyl ketone or Cellosolve Acetate. Application of the coating is done using a brush, or alternatively, by spraying methods. Of course, the fumes in the application area are to be well controlled.
After the application of the coating, the coated object should be kept in a well ventilated area until all solvent evaporates and the system cures completely. Alternatively, a faster curing can be achieved by using slightly elevated temperatures such as up to about 40xc2x0 C. to 60xc2x0 C., in a properly ventilated oven.